The goals of this application for continuation funding are: to deepen significantly our understanding of how a speaker's auditory acuity influences his or her speech motor planning; to describe the speech percep- tion and production of hearing-impaired adults and the effects of cochlear prostheses; and to evaluate and help refine a quantitative model of the role of hearing in speech. We pursue these goals by conducting ex- periments with normal-hearing speakers, and with postlingually deafened adults who receive cochlear im- plants. Our experiments measure the effects of the implants on speech in recordings made before implanta- tion and up to two years after, as speakers' auditory acuity evolves. According to our model of the role of hearing in adult speech motor control, many of the goals of speech movements are in the auditory domain. Consequently, a central theme of this research is the role of auditory perception in the feedback and feed- forward control systems that are used to achieve auditory goals during speech. Feedforward control is al- most entirely responsible for generating articulatory movements in adults. However, when there is a mis- match between the speaker's intention and the resulting auditory feedback during production of a speech sound, that error leads to corrective motor commands that serve to update feedforward commands for sub- sequent movements. The speaker's ability to detect such a mismatch depends on his or her auditory acuity. The proposed research inquires into the role of auditory acuity when producing phoneme and lexical stress contrasts; when compensating for feedback perturbation of vowel formants or for mechanical pertur- bation of sibilant spectra; and when imitating synthesized vowels. To measure acuity we present synthetic speech continua for discrimination testing. To assess relations with acuity, we measure the degree of sepa- ration of contrastive phonemes and lexical stress; dispersion of productions around their phoneme means; compensation for formant shift and for mechanical perturbation of sibilant spectra; and imitation accuracy. In most of these experiments we also block auditory feedback temporarily in order to reveal the state of feed- forward commands. Analyses take demographic variables such as age at hearing loss and duration of im- plant use into account. Relevance to public health: Hearing oneself speak plays a twofold role in maintaining good speech - it may correct the pronunciation of speech sounds even as they are occurring and it can revise plans for pro- ducing the sound next time. When a speaker is seriously hearing-impaired, those two mechanisms do not function well and speech can deteriorate over time. This project studies the idea that the people most af- fected are those who cannot distinguish the different speech sounds well. The results can serve to stimulate research on new therapies in which certain speech disorders, including those of speech development, are alleviated by training clients to hear differences better among the sounds that they produce.